Photodisc/Thinkstock(NEW YORK) -- The Fast Diet is a bestselling book that outlines a weight-loss plan that’s raising some eyebrows. In one week, the plan says, participants can eat what they want for five days, but must fast for two non-consecutive days.

Dr. Michael Mosley, the book’s author, defines fasting as 500 calories a day for women and 600 for men. Adults can consume these calories, which amount to 25 percent of a normal adult’s daily food intake, in one mid-day meal or spread across the course of the day. But protein and lots of fruits and vegetables are highly recommended.

The other five days supposedly require little willpower. Backers of this diet claim fast dieters can eat whatever they want, including cheeseburgers and croissants, in a guilt-free way.

Mosley said he never set out to create the next diet craze. In an interview with ABC News’ Nightline, Mosley said he was inspired to create the diet after a wakeup call from his own doctor.

“I had a bit of a nasty shock because I discovered that my fasting glucose levels were those of a diabetic,” Mosley, who lost his father to diabetes, said. “And my cholesterol levels [were] about twice [what] they should be.”

Mosley -- who will on April 3 appear in the PBS special Outlining the Science Behind ‘The Fast Diet’ -- said he lost nearly 20 pounds in two months on his Fast Diet and his overall health improved.

“My body fat went down from 28 percent to 20 percent, and my blood glucose went down from diabetic to normal,” he said. “My cholesterol went down from needing medication to normal.”

He also said he lost the fat in his gut, significantly reducing his risk of diabetes and heart disease.